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John has mostly managed to tune out the sound of Mycroft and Sherlock bickering. It's not exactly new and he knows that involving himself in the conversation won't help at all. He knows that much. As long as nothing's on fire, or melting when they're finished it counts as a good day. After all, it's not like they're going to come to blows - though it occurs to him that if the conversation goes badly they could do so much worse than that. So very much worse.

John stares at his laptop instead, and pretends their quiet sniping and their (not even veiled any more, really) insults are background noise.

"John," Sherlock says, strident, wheedling.

John's actually glad he wasn't paying attention.

"I'm not getting in the middle of you two, you can't even look at each other for five minutes without starting some sort of war and it's exhausting and I'm in no way equipped to deal with it." He attempts to look very busy in the hopes that they'll forget about him again. Because no one should get between those two, they'd be crushed within seconds, like atoms in a Supercollider. Which doesn't sound like fun at all. Not even a little. Not even for science.

It's all gone suspiciously quiet though - he risks turning around. Mycroft and Sherlock still seem to be fighting. Which is ridiculous, because they aren't talking, or even looking at each other now. No, they're looking at him instead. He frowns and wonders if he should be worried - then promptly worries anyway.

"What?" he demands, when he can't take it any more.

"Oh," Mycroft says quietly.

It's only one word but Sherlock turns his head toward him, looks briefly confused, as if Mycroft has just rattled off some sort of complicated formula for cold fusion. But then Sherlock raises an eyebrow in John's direction, focused, questioning, half a dozen things John can't even hope to understand. Whatever shows on his own face is apparently incriminating because Sherlock blinks like he's genuinely surprised at whatever he finds. John barely has time to worry about exactly what he's been giving away all over the place before Sherlock's eyes slide briefly sideways, then he sighs.

"Fine," he says to Mycroft, which is utterly bewildering.

Until Mycroft's hand lifts, fingers sliding up the sharp edge of Sherlock's jaw, easing him round to face him.

John inhales and goes still.

They have a brief silent conversation - which Mycroft apparently wins, because Sherlock slowly, reluctantly, agrees. It's in the faint, stiff twitch of his jaw. Mycroft takes one step forward. They're almost the same height, all it takes is one tilt, a raised eyebrow, a noise that's briefly considering. John has just enough time to think Mycroft absolutely won't do what he thinks he's going to do.

But then he does.

And Sherlock stays exactly where he is and lets Mycroft kiss him.

John's expecting some sort of angry protest. Sherlock angrily protests everything, doubly so when it comes to Mycroft. Though only Mycroft gets that special dramatic air of petulance that sibling rivalry seems to bring out in everyone. John's expecting anger, accusation, something. But instead the kiss opens out, a perfectly synchronised shift that leaves it suddenly nothing like chaste. Far past anything that siblings are supposed to share.

John drags a startled breath and nearly chokes on it when it rattles through his completely dry throat.

Oh Christ.

His open laptop hits the floor with a crack. A sound the both of them ignore in favour of attempting to find some sort of strange equilibrium in a kiss they're both trying to control. Age apparently wins, and there's a long hand tilting Sherlock's face just slightly. Just enough, and then Mycroft has his tongue in Sherlock's mouth and John feels like he's just been punched in the gut. It's a thousand different flavours of wrong. But Sherlock just grunts and takes it.

John's jeans are suddenly far too tight - all hope of accomplishing work-related tasks completely gone. He's more than a little ashamed of his own reaction. Because he didn't think he was the sort of person to encourage incest but Sherlock and Mycroft don't just break the rules, they set the pieces on fire. Leave the remains unrecognisable. This is insane, it's insane and so very, very wrong and he can't quite get enough air with them doing that, still doing - fuck.

No one should be able to look smug while kissing someone but Mycroft manages it. John had been fairly certain he'd never see Mycroft kiss anyone, ever. The fact that he's kissing Sherlock, six feet away, has stolen all rational thought from him.

He swears, audibly, shakily, and there's no hope at all of hiding the sheer amount of shocked want in his voice.

The kiss isn't tidy any more, Sherlock's mouth shines wet, never shut for long. He still isn't resisting, though his hands are loose at his sides, fingers shifting like they're not quite sure what they should be doing.

John can hear himself breathing, every hot, quick rush of it.

He honestly doesn't know how much of it is an act and how much of it is genuine. There's enough animosity there to fuel - whatever this is. But John feels vaguely ashamed again for thinking it. The alternative, that it's purely for him, that they are essentially performing for his benefit - that they knew in some way, things there is no way he would ever admit to. He's not sure if that's better or worse. It feels...too big to think about. Like even this is too much, something he shouldn’t be allowed to watch, never mind thinking about what else - if they would do anything else - and his brain tells him that if he goes there he'll lose any ability to think at all.

And on that, they part, mutually, cleanly. Sherlock still has one eyebrow raised, like he's mentally rating his brother's performance and that's about as much as John can take.

He manages to push himself to his feet, feeling like an odd sort of observer to some sort of strange science.

"I'm going upstairs," he manages over the hammering of his own pulse. He thinks he has just enough blood left in his legs to make them work. He's not a genius but he's fairly sure they're both going to follow him.